In well stimulation, the ability to perforate multiple zones in a single well and then fracture each zone independently, referred to as “zone fracturing”, has increased access to potential reserves. Zone fracturing helps stimulate the well by creating conduits from the formation for the hydrocarbons to reach the well. Many gas wells are drilled for zone fracturing with a system called a ball drop system planned at the well's inception. A well with a ball drop system will be equipped with a string of piping below the cemented casing portion of the well. The string is segmented with packing elements, fracture plugs and fracture plug seat assemblies to isolate zones. A fracture plug, such as a ball or other suitably shaped structure (hereinafter referred to collectively as a “ball”) is dropped or pumped down the well and seats on the fracture plug seat assembly, thereby isolating pressure from above.
Typically, in ball drop systems a fracture plug seat assembly includes a fracture plug seat having an axial opening of a select diameter. To the extent multiple fracture plugs are disposed along a string, the diameter of the axial opening of the respective fracture plug seats becomes progressively smaller with the depth of the string. This permits a plurality of balls having a progressively increasing diameter, to be dropped (or pumped), smallest to largest diameter, down the well to isolate the various zones, starting from the toe of the well and moving up.
A large orifice through an open seat is desired while fracing zones below that seat. An unwanted consequence of having seats incrementally smaller as they approach the toe is the existence of pressure loss across the smaller seats. The pressure loss reduces the efficiency of the system and creates flow restrictions while fracing and during well production.
In order to maximize the number of zones and therefore the efficiency of the well, the difference in the diameter of the axial opening of adjacent fracture plug seats and the diameter of the balls designed to be caught by such fracture plug seats is very small, and the consequent surface area of contact between the ball and its seat is very small. Due to the high pressure that impacts the balls during a hydraulic fracturing process, the balls often become stuck and are difficult to purge when fracing is complete and the well pressure reverses the flow and produces to the surface. If a ball is stuck in the seat and cannot be purged, the ball(s) must be removed from the string by costly and time-consuming milling or drilling processes.
FIG. 1 illustrates a prior art fracture plug seat assembly 10 disposed along a tubing string 12. Fracture plug seat assembly 10 includes a metallic, high strength composite or other rigid material seat 14 mounted on a sliding sleeve 16 which is movable between a first position and a second position. In the first position shown in FIG. 1, sleeve 16 is disposed to inhibit fluid flow through radial ports 18 from annulus 20 into the interior of tubing string 12. Packing element 24 is disposed along tubing string 12 to restrict fluid flow in the annulus 20 formed between the earth 26 and the tubing string 12.
FIG. 2 illustrates the prior art fracture plug seat assembly 10 of FIG. 1, but with a ball 28 landed on the metallic, high strength composite or other rigid material seat 14 and with sliding sleeve 16 in the second position. With ball 28 landed on the metallic, high strength composite or other rigid material seat 14, fluid pressure 30 applied from uphole of fracture plug seat assembly 10 urges sliding sleeve 16 into the second position shown in FIG. 2, thereby exposing radial ports 18 to permit fluid flow therethrough, diverting the flow to the annulus 20 formed between the earth 26 and the tubing string 12.
As shown in FIGS. 1 and 2, the metallic, high strength composite or other rigid material seat 14 has a tapered surface 32 that forms an inverted cone for the ball or fracture plug 28 to land upon. This helps translate the load on the ball 28 from shear into compression, thereby deforming the ball 28 into the metallic, high strength composite or other rigid material seat 14 to form a seal. In some instances, the surface of such metallic, high strength composite or other rigid material seats 14 have been contoured to match the shape of the ball or fracture plug 28. One drawback of such metallic, high strength composite or other rigid material seats 14 is that high stress concentrations in the seat 14 are transmitted to the ball or fracture plug 28. For various reasons, including specific gravity and ease of milling, balls or fracture plugs 28 are often made of a composite plastic or aluminum. Also, efforts to maximize the number of zones in a well has reduced the safety margin of ball or fracture plug failure to a point where balls or fracture plugs can extrude, shear or crack under the high pressure applied to the ball or fracture plug during hydraulic fracturing operations. As noted above, when the balls 28 extrude into the metallic, high strength composite or other rigid material seat 14 they become stuck. In such instances, the back pressure from within the well below is typically insufficient to purge the ball 28 from the seat 14, which means that an expensive and time-consuming milling process must be conducted to remove the ball 28 from the seat 14.
Other prior art fracture plug seat assembly designs include mechanisms that are actuated by sliding pistons and introduce an inward pivoting mechanical support beneath the ball. These designs also have a metallic, high strength composite or other rigid material seat, but are provided with additional support from the support mechanism. These fracture plug seat assembly designs can be described as having a normally open seat that closes when a ball or fracture plug is landed upon the seat. Such normally open fracture plug seat assembly designs suffer when contaminated with the heavy presence of sand and cement. They also rely upon incrementally sized balls so such systems suffer from flow restriction and require post frac milling.